1. Field of the Invention
The invention relates to a portable communication device, an in-vehicle communication device, and a communication system incorporating said communication devices. In particular, the invention relates to processes for filtering the signals received by an in-vehicle communication device.
2. Description of the Related Art
In recent years, various technologies have been developed in which specific information on a vehicle (e.g., motor vehicle), such as information on the pressures of the tires of the vehicle, is regularly detected and the detected vehicle information is indicated to the driver or a warning is given to the driver based on the detected vehicle information while controlling the vehicle based on the detected vehicle information. As one example of such technologies, Japanese Patent Application Publication No. 2002-337521 (JP-A-2002-337521) describes a tire pressure estimation system. According to this system, the pressures of the tires are estimated from the rotation speeds of the wheels based on a specific relation between the resonance frequency of the wheel rotation speed and the tire pressure. However, external disturbances that are white noises are input to the tire-suspension system of the vehicle, resulting in noises on the signals carrying the information on the rotation speeds of the respective wheels of the vehicle. According to the technology of JP-A-2002-337521, therefore, it is necessary to remove the noises on the wheel speed signals when extracting the resonance frequency components used to estimate the tire pressures. For this reason, in the technology of JP-A-2002-337521, multiple filtering processes are executed using a plurality of filters having different cut-off frequencies, and the resonance frequency components are extracted based on the results of the filtering processes, and the tire pressures are estimated based on the resonance frequency components thus extracted.
According to the technology of JP-A-2002-337521, however, even if the resonance frequency components can be exclusively extracted, the obtained tire pressures are only estimated values, and therefore there is a possibility that the estimated tire pressures are different from the actual tire pressures.
On the other hand, in other example, tire pressure sensors are provided at the respective wheels of the vehicle to directly detect the pressures of the respective tires, and information regarding the detected tire pressures is transmitted to a receiver by radio. That is, in this case, sensors are provided at detection target objects for obtaining specific vehicle information, and the vehicle information directly obtained via the sensors is transmitted by radio.
Further, a technology is known which enables extracting target frequency components through only one filtering process, not multiple filtering processes. According to this technology, an adaptive filter that filters signals using a specific filter coefficient is used.
An adaptive filter filters input signals while continuously updating its filter coefficient so as to remove all the frequency components including noises from the input signals. Therefore, in a case where an adaptive filter is used to extract signals transmitted by radio and carrying specific vehicle information, such as tire pressures, the process for updating the filter coefficient of the adaptive filter needs to be suspended when extracting said vehicle information signals. If it is not suspend when extracting the vehicle information signals, it results in the filter coefficient of the adaptive filter being updated so as to remove also the vehicle information signals that should not be removed. To suspend the filter-coefficient updating process when extracting the vehicle information signals, for example, the filter-coefficient updating process may be suspended when the in-vehicle receiver detects that vehicle information signals have been received.
However, it is difficult to keep powering the sensors provided at the detection target objects in motion, which are, for example, tire pressure sensors provided at the respective tires. Therefore, in this case, for example, small batteries are provided at the respective tire pressure sensors to power them. However, because the amount of power supplied from each battery is very limited, and the power consumption of the sensors needs to be reduced. For example, the power consumption of each sensor may be reduced if the information on the detected tire pressure is regularly transmitted by radio at given time intervals, rather than being continuously transmitted. In this case, the power used to transmit the vehicle information signals to the in-vehicle receiver is relatively small.
In the above-described case, however, if the vehicle information signals are asynchronously transmitted from the respective sensors, which are provided at the detection target objects, by radio at given time intervals, that is, without being synchronized, there is a possibility, although small, that the signals are superimposed. If the in-vehicle receiver receives such superimposed signals, it can not accurately identify each vehicle information signal. In the above-described case, therefore, the process for updating the filter coefficient for the adaptive filter can not be suspended when superimposed signals are received, and as a result, the filter coefficient for the adaptive filter is updated so as to remove the superimposed signals from the signals received by the in-vehicle receiver, which is not desirable. If the in-vehicle receiver filters the received signals using the filter that removes superimposed signals, the vehicle information signals that are not superimposed are also removed.